Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 398,565	$ 392,734	$ 386,556	$ 384,807	$ 366,082	$ 360,165	$ 355,356	$ 355,246
Operating costs and expenses	368,122	363,522	357,875	355,390	355,667	332,090	329,341	330,699
Income before income taxes	30,529	30,571	29,683	29,604	11,712	26,741	26,257	25,054	$ 120,387	$ 89,764	$ 31,708
Net income	$ 20,299	$ 19,711	$ 18,760	$ 18,626	$ 9,134	$ 17,086	$ 16,288	$ 15,516	$ 77,396	$ 58,024	$ 21,850
Basic earnings per common share (in dollars per share)	$ 0.28	$ 0.27	$ 0.26	$ 0.26	$ 0.13	$ 0.24	$ 0.23	$ 0.22	$ 1.06	$ 0.81	$ 0.31
Diluted earnings per common share (in dollars per share)	0.28	0.27	0.26	0.26	0.13	0.24	0.23	0.22	1.05	0.80	0.31
Cash dividends declared per common share (in shares)	$ 0.18625	$ 0.185	$ 0.18375	$ 0.1825	$ 0.18125	$ 0.18	$ 0.17875	$ 0.1775	$ 0.7375	$ 0.7175	$ 0.6975